The present invention relates to an apparatus for controlling the pressure in an ink reservoir of an ink jet printer, comprising a casing and a piston movable relative to the casing and defining therewith a variable-volume chamber communicating with the ink reservoir, the piston being biased to maintain a pressure difference between the variable-volume chamber and the outside.
An ink jet printer typically comprises printhead having one or more nozzle arrays and an ink reservoir from which liquid ink is supplied to the nozzles of the nozzle arrays, so that ink droplets may be ejected from the nozzles by thermal or piezoelectric action, as is generally known in the art. When the level of ink in the ink reservoir is higher than the level of the nozzles, the ink reservoir should be maintained at subatmospheric pressure in order to avoid ink from leaking out through the nozzles. Since the difference between the internal pressure in the ink reservoir and the atmospheric pressure has an influence on the process of droplet generation and hence on the quality of the printed image, it is desirable to keep this pressure difference constant. Since, however, the internal pressure in the ink reservoir may vary in response to changes in the ink volume contained therein, thermal expansion and the like, it is necessary to control the internal pressure in the ink reservoir.
U.S. Pat. No. 5,039,999 discloses a pressure control apparatus of type indicated above, in which a coil spring is employed for biasing the piston. U.S. Pat. No. 4,509,062 discloses another type of pressure control apparatus in which the variable-volume chamber is bounded by an elastically deformable bladder.
Both conventional designs have the drawback that the elastic biasing forces which maintain the pressure difference between the internal pressure in the ink reservoir and the atmosphere depend on the amount of deformation of the spring or the bladder, respectively, so that the pressure difference may still vary along with the expansion or contraction of the variable-volume chamber.
EP-A-0 375 383 describes a pressure control apparatus in which the variable-volume chamber is partly bounded by a rolling diaphragm. This rolling diaphragm provides a substantially linear volume/pressure characteristic, similar to that of a piston biased by a coil spring. In this apparatus, the rolling diaphragm is used only for mitigating the pressure fluctuations in response to volume changes, and the pressure is ultimately maintained constant by sucking air bubbles or liquid into the variable-volume chamber through a small orifice. Thus, this apparatus requires a rather complicated design and further has the problem that slight pressure fluctuations are induced by the air bubbles sucked into the variable-volume chamber.
It is an object of the present invention to provide a simple apparatus which maintains the internal pressure in the ink reservoir at a constant value with high accuracy.
According to the present invention, this object is achieved with an apparatus in which the piston is biased mainly by gravitational forces.
Since the gravitational forces which may be created by the weight of the piston itself or by an additional load applied thereto are constant irrespective of any changes in the volume of the variable-volume chamber, the internal pressure in the ink reservoir, or more exactly, the pressure difference between the ink reservoir and the outside, can be maintained constant with high accuracy even when the volume of the variable-volume chamber is allowed to vary within a comparatively large range. As a result, a high quality of the printed image can be achieved even with a system in which the droplet generation process is highly sensitive to the pressure drop across the nozzles, and the ingress of air into the nozzles is safely prevented. The apparatus according to the present invention is particularly useful in combination with a hot-melt ink jet printhead which is operated at elevated temperatures.
Preferably, the piston is connected to the walls of the casing defining the variable-volume chamber by means of a highly flexible diaphragm which provides a perfect seal for the gap between the piston and the walls of the casing without inducing any substantial friction between the piston and the casing. As a result, the frictional effects are negligible, even in the case where the internal pressure in the ink reservoir is only slightly below atmospheric pressure, e.g. in the order of 1 kPa below atmospheric pressure, and the effective pressure-sensitive area of the piston is comparatively small, wherein the gravitational forces involved in biasing the same are extremely small.
In a particularly preferred embodiment, the casing defining the variable-volume chamber has the form of a cylinder, and the piston is fitted therein with a small annular gap formed between the outer circumferential surface of the piston and the inner circumferential surface of the cylinder walls, and the rolling diaphragm is accommodated in this annular gap. In this way, a particularly compact construction of the apparatus is achieved, and the diaphragm is smoothly and stably guided in the cylinder without any substantial friction. Since the diaphragm is not subject to any substantial tensile stresses, it can be made extremely thin so that to will not exert any elastic forces on the piston. The pressure difference between the inside and the outside of the variable-volume chamber will help to keep the two layers of the rolling diaphragm apart, and since, when the piston is displaced, relative movement occurs only between the two layers of the diaphragm, friction is almost completely eliminated. In addition, since the diaphragm is not required to have elastic properties, the material may be optimized in view of reducing its frictional coefficient relative to itself.
While the printhead of an ink jet printer is generally mounted on a moving carriage, the pressure control apparatus can be mounted on a stationary frame of the printer and can be connected to the ink reservoir of the printhead through a flexible hose. Thus, the weight-biased piston will not be subject to any substantial forces of inertia. If the printhead comprises a plurality of ink reservoirs, for example in a color printer, all ink reservoirs may be connected to same pressure control apparatus.
Due to the constant gravitational forces acting upon the piston, the displacement of the piston depends linearly on the total air volume in the ink reservoir, the variable-volume chamber and the hose connection therebetween. In the long run, this air volume tends to slightly increase due to leakage or diffusion. In a preferred embodiment, this increase of the air volume is detected by monitoring the position of the piston, so that a reset process for evacuating the ink reservoir can be initiated automatically, when necessary. Likewise, the displacement of the piston can be used for generating a signal for automatically terminating the evacuation process when the air volume has again reached its target value. As an alternative, the variable-volume chamber is reset to a specific volume at regular intervals, for example at the end of each scan cycle of the printer.